1. Field of the Invention
This invention relates to an apparatus for forming copies through the steps of scanning an original and projecting the optical image of the original upon a movable electrophotographic photosensitive member, and more particularly to an electrophotographic apparatus capable of copying an original selectively at different magnifications.
2. Description of the Prior Art
Those of variable magnification electrophotographic copying apparatus of the original scanning type which have been put into practical use are constructed so that the velocity of a photosensitive member is the same for any copying magnification while the original scanning velocity is changed correspondingly to a selected magnification. Such apparatus have an advantage that copies can be obtained at the same speed for any copying magnification because the velocity of the photosensitive is invariable, while they also have various disadvantages. For example, in an apparatus wherein the one-to-one magnification copying at a magnification 1 and the reduced-scale copying at a magnification m (m&lt;1) are possible, if the velocity of the photosensitive member is v for any magnification, the original scanning velocity during the one-to-one magnification copying is v but the original scanning velocity during the reduced-scale copying is v/m. That is, the original scanning velocity during the reduced-scale copying is greater than that during the one-to-one magnification copying. The most widely used original scanning device is of the type in which an original supporting table or mirrors are reciprocally moved along a straight guide path, but in such device, countermeasures must be taken against the shock and/or vibration of the device occurring at the time of starting or stoppage, because such shock and/or vibration would induce noise or failure of the copying apparatus and would also cause blur of the image. On the other hand, the above-described shock and/or vibration becomes greater as the scanning velocity is higher and therefore, the means for preventing such shock and/or vibration becomes bulky and also leads to increased cost.
Also, for example, in an apparatus wherein the one-to-one magnification copying at a magnification 1 and enlarged-scale copying at a magnification m' (m'&gt;1) are possible, the original scanning velocity during the enlarged-scale copying is v/m' which is lower than that during the one-to-one magnification copying, but under-exposure occurs because the velocity of the photosensitive member is invariable. If the aperture of the lens is opened to prevent this, flare tends to occur to the image and, if the brightness of the original illuminating lamp is enhanced, there occurs an inconvenience that the amount of electric power consumed is increased. Also, the amount of under-exposure is not linearly varied for a variation in magnification and therefore, very much complicated means would be required to eliminate the under-exposure simply by adjusting the aperture and the lamp brightness.
In short, even a variable magnification copying apparatus is most frequently used for the one-to-one magnification copying. Accordingly, it would be most rational to construct the apparatus with the mechanical, electrical and physical requirements required for the one-to-one magnification copying being taken as the standard. However, in an apparatus wherein the velocity of the photosensitive member is the same for any copying magnification, there are many unreasonable or useless parts as viewed from the viewpoint of the one-to-one magnification copying and this has led to the bulkiness and complication of the apparatus as well as increased cost and deterioration of copy images. It has also led to a smaller degree of freedom with which the apparatus is constructed.
On the other hand, if design is made such that the velocity of movement of the photosensitive member is changed correspondingly to a change in copying magnification, the bulkiness and increased cost of the apparatus could be avoided and the construction of the apparatus having a reduced number of unreasonable or useless parts would become possible with the various requirements for the one-to-one magnification copying being taken as the standard, but if the velocity of movement of the photosensitive member is simply changed without any contrivance, deterioration of copy images would occur. This is because the speed at which image is formed on the photosensitive member is varied for each selected magnification.